This invention relates to pressure sensitive adhesives based on aqueous emulsions and processes for preparation of the adhesives. The pressure sensitive adhesives of the invention have enhanced resistance to water-whitening.
These pressure sensitive adhesives are particularly suitable for applications that require that the pressure sensitive adhesive maintain adhesion between the substrate and facestock when subjected to hot water spraying or immersion. Hot water adhesion is required in applications such as bottle labels where the bottles are subjected to hot water spraying in washing operations. In general, resistance to water-whitening is desirable anywhere a pressure sensitive adhesive with transparent facestock or substrate is subjected to water or high humidity. Examples include labels on the sides of trucks, signs and bottles.
Methods of providing water-whitening resistant latex emulsions for use in pressure sensitive adhesives are disclosed in the art. U.S. Pat. No. 5,286,843 discloses a process for improving the water-whitening resistance of pressure sensitive adhesives containing an aqueous latex emulsion and water soluble ions by removing the water soluble ions and adjusting the pH to at least about 6. The patent discloses that water soluble ions may be removed by a number of techniques including centrifugation, dialysis, precipitation and deionization with ion exchange resins. The preferred method of removing the water soluble ions is to contact the aqueous latex emulsion, the formulated pressure sensitive adhesive containing the aqueous emulsion or both with an ion exchange resin. International Publication No. WO 97/11996 A1 discloses a process for preparing hot water-whitening resistant latex emulsions useful in pressure sensitive adhesive compositions. The process involves copolymerizing a monomer mixture containing at least one alkyl acrylate ester of an alcohol containing at least 4 carbon atoms, at least one polar co-monomer, and at least one partially water-soluble co-monomer present in an amount of at least about 7 wt. %. Polymerization is carried out in the presence of at least one nonionic surfactant containing at least 8 moles of ethylene oxide and at least one anionic surfactant containing up to about 10 moles of ethylene oxide. The polymerization product is neutralized to produce an emulsion having a pH greater than 7 and containing particles having a volume average particle size diameter up to about 165 nm. An electrolyte may be added subsequent to polymerization to stabilize opacity of a film cast from the emulsion. International Publication No. WO 00/61670 A1 discloses a process for preparing an aqueous latex emulsion for use in pressure sensitive adhesives that maintain adhesion in hot water environments in addition to exhibiting enhanced resistance to water-whitening. The process involves preparing an aqueous latex emulsion from a monomer mixture consisting essentially of at least one alkyl acrylate having at least 4 carbon atoms in the alkyl chain, at least one ethylenically unsaturated carboxylic acid, and at least one styrenic monomer, the polymerization being carried out in the presence of at least one anionic surfactant and a redox type free radical initiator system. The aqueous latex emulsion prepared according to the process of WO 00/61670 is disclosed to have polymer particles with a mean particle size diameter of less than or equal to about 100 nm. None of the above patents/publications disclose preparing pressure sensitive adhesives wherein a polymerizable surfactant is used.
U.S. Pat. Nos. 5,928,783 and 6,239,240 disclose processes for preparing aqueous emulsion polymers that have application as pressure sensitive adhesives. The polymers are prepared by reacting at least one ethylenically unsaturated monomer and a polymerizable surfactant having a terminal allyl amine moiety. Neither patent discloses preparation of pressure sensitive adhesives having enhanced resistance to water-whitening.
International Publication No. WO 01/85867 A1 discloses aqueous blush-retardant pressure sensitive adhesives made from an aqueous latex emulsion having an average particle size of not substantially above about 100 nm and emulsified in the presence of a reactive emulsifier consisting essentially of alkyl, alkenyl or aralkyl substituted phenyl compounds having at least one propenyl substituent, and a polyoxyalkylenesulfonate salt substituent as taught in U.S. Pat. No. 5,332,854. The aqueous latex emulsion is prepared from a monomer mixture consisting essentially of about 50 to about 90 wt. % of at least one acrylate having at least 4 carbon atoms in the alkyl chain, about 5 to about 10 wt. % of at least one ethylenically unsaturated carboxylic acid or its corresponding anhydride, and about 15 to about 40 wt. % of at least one styrenic monomer, wherein some or all of the styrenic monomer can be replaced with a hard monomer, i.e. a monomer having a Tg greater than 30° C. such as methyl methacrylate, isobornyl acrylate, vinyl acetate, and the like.